This invention relates to a method for correcting deviations in a surface that are created by manufacturing processes. More particularly, this invention relates to a method for correcting optical surfaces having such imperfections. Still more particularly, this method relates to a method for filling in lap marks created during the manufacture of aspheric reflective optical surfaces to create a near perfect optical surface.
Although the process of this invention has been reduced to practice in the context of reflective aspheric optical surfaces useful in extreme ultraviolet lithography processes using 13.4 nm radiation, this process is applicable to the correction of imperfections in a wider realm of surfaces where a near perfect surface is necessary.
There are several technologies for correcting the wavefront of optical surfaces including: sub-aperture polishing, ion milling, through a mask with a single large hole, and binary optics. Sub-aperture computer controlled polishing produces the best aspheric optical surfaces presently available. This process starts with a near perfect spherical surface produced by a conventional technique followed by controlled polishing with a small (sub-aperture) lap to accommodate the deviation from a sphere. With such a technique, the aspheric surface can be polished in, and low-spatial-frequency errors such as third-order astigmatism and coma can be polished out. The very best resulting mirror surfaces have RMS roughnesses of about 1 nm composed of mid-spatial frequency aberrations--an unfortunate side effect of using a small polishing lap. These are the errors the present technique would remedy; they are largely uncorrelated "valleys" and "hills" with characteristic widths that range from 2 mm to 30 mm on a 100 mm substrate.
Ion milling can be used to level high spots on an optical surface. However, it leaves an optically rough surface because of grain boundaries and subsurface damage. Research is on-going to ameliorate these subsurface damage effects.
Mild aspheres are being made commercially using coating processes similar to the present process. However, the masks used in these commercial processes contain only a single large aperture or a relatively few apertures that do not provide adequate deposition overlap for the high accuracy corrections produced by the present invention. These apertures are designed so the deposition is heavier where the parent surface needs to be built up. This technique works well when adding low-order spherical aberration to a surface, but it cannot be used to correct high order aberrations or mid-spatial frequency errors. These depositions were very much thicker (hundreds of microns) than those contemplated herein. These thick depositions create a surface roughness level, about 1% RMS of the deposition thickness, that is unacceptable for many applications.
Finally, the use of binary optics can correct high order aberrations and mid-spatial frequency errors, however it is only useful for monochromatic applications. Furthermore, although it is a known technology and can reasonably be employed when the discrete steps remaining in the finished surface are not a problem, the steps resulting from binary optics correction scatter a minimum of 1-2% of the light which would be a problem when there are bright objects in the field and one needs to see faint ones. It may also cause trouble during the application of multi-layer reflective coatings for very short wavelengths, which is common practice.
The optical surface optimization process of this invention appears to be the only option for the highest performance systems operating at extremely short wavelengths, far below the visible region of the electromagnetic spectrum. This process will enable the manufacture of diffraction-limited optical systems for UV, extreme UV and soft X-ray spectral regions, which would have great impact on photolithography and astronomy. It is also applicable for use in the visible wavelength region.